1. Field of the Invention
The present invention relates to an image processing technique for an X-ray image.
2. Description of the Related Art
There is a procedure called angiography for extracting a structure such as a blood vessel that cannot be seen by ordinary X-ray imaging. Angiography is an imaging method of extracting a blood vessel by injecting a medicine called a radiopaque dye into the blood vessel and performing X-ray imaging.
As this angiography, there is an image processing method called digital subtraction angiography (DSA) that extracts only a blood vessel by removing structures except for the blood vessel by performing a subtraction process between an image before a radiopaque dye is injected and an image after the radiopaque dye is injected.
A DSA image as an image obtained by this DSA has the problem that the subtraction process makes noise larger than that of an image before the subtraction. Japanese Patent Laid-Open No. 11-47123 has disclosed a technique that reduces noise by performing tone processing by which the contrast of a pixel value close to 0 decreases.
When reducing noise by this tone processing, it is necessary to give proper tone conversion characteristic data. If tone processing is performed so as to broaden a contrast decreasing range in order to increase the DSA image noise reducing effect, the contrast of even an angiography region sometimes decreases. On the contrary, if the contrast decreasing range is narrowed, it is sometimes impossible to obtain a sufficient noise reducing effect.